In the production of bottles in polymer material a method is known for making plastic bottles starting from a preform in plastic or polymer material comprising a pre-heating step of the preform at a predefined process temperature, a transfer step of the preheated preform to a mould and insertion therein, and a blowing step inside the preform positioned in the mould in a closed position, of air at a predetermined pressure so that the preform is inflated inside a cavity made in the mould, shaped like the bottle to be obtained, until the preform, inflated by said pressurised air, adheres to the walls of the cavity, cooling upon contact and stiffening, thereby acquiring the definitive shape of the bottle to be obtained. As well as the blowing step described, there may also be a step of stretching the preform when softened, suitable for elongating or mechanically extending the preform being prepared and simultaneously with the blowing. Such combined method is called stretch-blowing.
The above method is performed by automatic machines which, to ensure a greatly reduced unit cost, must implement a very high hourly production rate.
To maximise efficiency, the preforms need to be moved at high speed along a circuit between the output of a preheating furnace and the mould, passing through one or more movement devices comprising the aforesaid feed device suitable for positioning one preform at a time in the mould.
Such circuit is generally formed of straight and curved sections having different curvatures and therefore speed variations of modulus and direction at the point of changes in direction, thereby causing damaging accelerations for the preheated preform.
The problem of acceleration is particularly felt in the passage of the preform from the feed device to the mould on account of the oscillatory movements which are triggered on the preheated, therefore softened preform, as a result of the aforesaid acceleration.
To better describe the aforesaid drawback of the prior art the preform and its behaviour along the aforesaid circuit must be described in more detail.
The preform has generally an elongated tubular shape having a first open end having a mouth and a second opposite closed end. The preform is transported hanging vertically, gripped at the mouth.
In the pre-heating furnace the preform is brought to the softening point, so as to be subsequently inflated inside the cavity in the mould.
It is evident that the preform, in the above conditions, being softened, reacts to the accelerations it is subjected to by gently swinging like a pendulum and deforming in relation to its mouth. The oscillation caused by an acceleration can be summed to a previous acceleration not yet damped producing an uncontrolled oscillation movement of the preheated preform. Such uncontrolled movement of the preform may persist even when the preform is inserted in the mould, with the risk of brushing against the mould in some points before the stretch-blowing step cooling and solidifying irregularly in that point and thereby giving rise to a bottle or container with moulding defects.
Returning to the known stretch-blowing systems, a widely used method is to use a stretch-blowing machine of the type rotating around to a vertical axis. Such machine generally comprises a plurality of moulds positioned radially in relation to the axis of the machine and provided with opening/closing means of the moulds, operable in a manner synchronised with the corresponding movement devices of the preform.
Sometimes such movement devices comprise a transferring star. A transferring star is taken to mean for example a support having a plurality of seats for receiving a preform positioned spaced tangentially around a rotation axis, in particular along a circumference with its centre in the rotation axis. Such transferring star is suitable for sequentially supplying the preforms to the mould of the blowing machine.
In the conventional machines, in the passage from the transferring star to the mould of the rotary machine, the preform may be subject to brusque speed variations back and forth, and thus an acceleration, with the drawbacks mentioned above.
The acceleration to which the preform is subjected is formed by a tangential component along the direction of advancement and a radial component orthogonal thereto.
While the known systems are generally able to keep the radial component of the acceleration at a low value, the value of the tangential component has often an high value, to the order of hundreds of m/s2.
The need is therefore felt to provide a handling system of a preform able to pick it up from a feed device and rapidly transfer it to a stretch-blow moulding position by movements entailing reduced acceleration values, in particular of the tangential acceleration component, so as to prevent uncontrolled oscillatory movements of the softened preform from being triggered.